This invention relates to solar collectors and in particular a focussing solar collector.
Focussing collectors can be divided broadly into three systems.
(a) Troughs PA1 (b) Dishes PA1 (c) Heliostat/power tower combinations PA1 (a) Tracking on one axis. PA1 (b) Tracking on two axes. PA1 (a) Parabolic PA1 (b) Spherical PA1 (a) Point focus PA1 (b) Immobile target (allowing direct generation of steam without the use of an intermediate exotic heat transfer fluid and a steam generator). PA1 (c) Ease of construction PA1 (d) Simple tracking
The first two systems can be further subdivided as follows:
Troughs
Dishes
The main problem with troughs is the relatively low maximum temperatures attainable because a line focus rather than a point focus is produced. This applies to both types although the second type is better in this regard than the first. Low temperatures cause heat transfer problems and give low thermodynamic efficiency. In endeavouring to overcome these problems by employing two axes tracking other complications arise namely extracting heat at high temperatures from the focus of a system which is swinging around as the sun is tracked. The hot, exotic heat transfer medium tends to leak past the seals in pipes undergoing relative movement. (The exotic heat transfer fluids are necessary because the pressure of steam at even the relatively low operating temperatures, typically 570.degree. F., is 1,200 p.s.i., which is enough to cause a blow out). This is apart from the actual tracking problem--the rate of swing on the two axes cannot be preset as it varies for each axis during the day, being a maximum for the elevation axis and a minimum for the azimuth axis at dawn and the reverse at noon, then back to the dawn situation at sunset. Furthermore, the changing relationship between the two axial swings changes over successive 24 hour periods during the course of 12 months. The usual tracking method is to adjust the elevation and azimuth at frequent intervals during the day, this being accomplished automatically via input from sensors. However, this results in a jerky movement and a lot of the time an appreciable percentage of the reflected incident radiation misses the target.
Parabolic dishes produce a point focus resulting in high temperatures but they suffer the other disadvantages of two axes troughs, namely a swinging focus, leaky exotic heat transfer fluid lines (exacerbated by the higher temperature) and tracking problems (exacerbated by the generally bigger unit size and the greater precision required). In addition, it is expensive to fabricate and support a large parabolic dish--each panel in the dish is different to every other not on its particular "latitude zone".
Spherical dishes are an improvement in many respects. For instance they can be fabricated out of a large number of a few types of panels and the tracking requirements are simple. A spherical dish has an infinity of optical axis, as opposed to a paraboloid which has one so that even if completely immobile it is, in a sense, always perfectly aligned with the sun. An immobile dish of course would only receive a fraction of the total possible energy intake during the day so in practice the dish is tilted and rotated around a vertical axis through the centre of curvature so that it is facing in the general direction of the sun at all times during the day. The trade off for these advantages is the loss of the point focus--the "non paraxial" incident rays (ie; those parallel but at some distance from the optical axis) are reflected to points on the axis progressively further and further from the paraxial focus as they become more and more "non paraxial". In section the reflected rays form a caustic curve. The result is that in order to collect the reflected radiation a heat transfer fluid pipe must at all times be positioned coincident with the optical axis of the moment. In other words the system reverts to a low temperature line focus with a swinging target. In this instance the target swing is indepedent of the reflector and is relatively straight forward--it rotates around a North-South axis through the centre of curvature inclined at the latitude angle of the site at the same speed as the earth rotates with respect to the sun which is essentially constant at 15.degree. per hour. It also swings on a second axis through the centre of curvature at right angles to the first axis to maintain the angle between the first axis and the pipe essentially equal to the declination of the sun as it changes with the seasons. This is known as an equatorial mounting and provides the simplest possible tracking.
The heliostat/power tower system is currently the most favoured in large solar power installations. It consists of a large central tower with an immobile target sitting on top. The tower is surrounded at ground level by a large number of plane reflecting surfaces which reflect sunlight to the target. This system has many inherent advantages but the tracking problem is multiplied by the number of individual reflectors used--each reflector has to track in elevation and azimuth and its attitude at any time is different to all the other reflectors. At least with two axes troughs and parabolic dishes all units have the same attitude at the same time--one unit can act as the master tracking unit which all the others follow using the servo principal.
Obviously one system is required that incorporates the good points of the various existing systems without the problems.
The desirable attributes are:
It has been found that this could be achieved by introducing a secondary reflecting surface into a spherical dish system which gives the spherical dish a point focus at the centre of curvature.